This invention relates to induction heating, and more specifically to improved induction heating apparatus which provides smooth power control responsive to operator commands.
Induction heating, accomplished by eddy currents circulating in the surface of a metallic object with a time variant magnetic field to generate joule losses therein, has several advantages over heating by conventional techniques such as convection or conduction. In contrast to convection or conduction heating, induction heating is usually faster because of the lower thermal mass associated with induction heating systems. Also, because induction heating concentrates heat within the material of the object, as opposed to conduction or convection heating which concentrates heat on the object surface, higher energy transfer efficiency is obtained.
One of the major impediments to implementing induction heating on a widespread basis has been the inability to achieve smooth control of the power supplied by the inverter to the induction drive coil generating the time variant magnetic fields which circulate the eddy currents in the object surface. Inverters such as the type described and claimed in copending application Ser. No. 107,259, entitled "Parallel Resonant Induction Cooking Surface Unit," filed by Steigerwald et al., on Dec. 26, 1979 and assigned to the General Electric Co., have heretofore been controlled by varying the inverter duty cycle or the output frequency. While control of the inverter duty cycle or inverter output frequency permits control of inverter output power over a limited range, such control methods do not permit inverter output power to be smoothly regulated between zero and one hundred percent of maximum output power.
Accordingly, one object of the present invention is to provide induction heating apparatus which permits smooth control of inverter output power between zero and one hundred percent of maximum output power.
Another object of the present invention is to provide induction heating apparatus which may be fabricated of inexpensive components.